Pixel circuit of organic light-emitting display

ABSTRACT

A pixel of an OLED display is disclosed. A gate voltage of a driving transistor can be precisely adjusted using a second gate electrode that can supply DC power easily securing an operation range of an OLED. Further, by only adding one power line that can precisely adjust a gate voltage of a driving transistor to an OLED display, an operation range of the OLED can be easily secured and thus a drain current can be reduced without increasing a channel length of the driving transistor resulting in a narrower pixel area. According to various embodiments, the pixel can secure an operation range of the OLED by reducing a magnitude of a drain current by adjusting a gate voltage of a driving transistor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of Korean PatentApplication No. 10-2013-0046361 filed in the Korean IntellectualProperty Office on Apr. 25, 2013, the entire contents of which areincorporated herein by reference.

BACKGROUND

Field

The described technology generally relates to a pixel circuit that isincluded in an organic light-emitting diode (OLED) display.

Description of the Related Technology

OLED displays are manufactured as a matrix of individual pixel circuits.Each pixel circuit includes an OLED that defines a pixel of a particularcolor. In order to have grayscales which define a range of color in anOLED, it is generally necessary to have a linear relationship ofluminance to current density in the OLED over a wide area of each pixel.

Generally, to have this relationship over a wide area of the pixel, itis necessary to enlarge an operation range of the OLED. To achieve thispurpose, lowering a slope of a drain current I_(d) graph in a saturationarea may be used.

In general, a drain current in a saturation area of a transistor isrepresented by Equation 1.

$\begin{matrix}{I_{d} = {\frac{1}{2}\frac{W}{L}\mu_{n}{C_{ox}\left( {V_{GS} - V_{TH}} \right)}^{2}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

Typically, drain currents have been reduced by increasing the channellength L of a driving transistor, largely providing a reasonable rangeof color in the OLED.

However, when a resolution of a display device increases, a size of apixel within the display device decreases and thus because the space apixel circuit can be disposed within decreases, it is difficult tolargely secure an operation range of the OLED using a method ofincreasing a channel length of a driving transistor.

Further, because a channel length cannot be increased with a real RGBmethod, another technique to lower a slope of a current curved lineshould be used.

The above information is designed to assist in understanding thedisclosed technology and therefore it may contain information that doesconstitute prior art.

SUMMARY OF CERTAIN INVENTIVE ASPECTS

One inventive aspect is a pixel circuit of an OLED display configured tosecure an operation range of an OLED by reducing a magnitude of a draincurrent by adjusting a gate voltage of a driving transistor.

Another aspect is an OLED display that generates light with an OLED. TheOLED display includes: a display unit including a plurality of pixelsincluding at least one thin film transistor (TFT), at least onecapacitor, and at least one OLED; a scan driver that transfers aplurality of scanning signals to the display unit; a light emissiondriver that transfers a plurality of light-emitting signals to thedisplay unit; a data driver that transfers a plurality of data signalsto the display unit; a DC power source unit that transfers DC power tothe display unit; and a controller that controls the scan driver, thelight emission driver, the data driver, and the DC power source unit,wherein a driving transistor that controls a driving current flowing tothe OLED among the at least one TFT includes a first gate electrode thatreceives a supply of a data voltage by one data signal of the pluralityof data signals and a second gate electrode that receives a supply of aDC voltage from the DC power source unit.

The first gate electrode and the second gate electrode may beelectrically insulated by a second insulation layer, and the second gateelectrode and a contact electrode that is connected to the first gateelectrode may include a driving transistor that is electricallyinsulated by a third insulation layer.

The OLED display may further include a driving transistor in which theDC voltage is higher than the data voltage and is supplied to the secondgate electrode, when the driving transistor is a p-MOSFET.

The OLED display may further include a driving transistor in which theDC voltage is lower than the data voltage and is supplied to the secondgate electrode, when the driving transistor is an n-MOSFET.

The scan driver may supply at least two scanning signals of theplurality of scanning signals to one pixel of the plurality of pixels,supply one data signal of the plurality of data signals to the one pixelaccording to a first scanning signal of the at least two scanningsignals, and supply an initialization voltage to the one pixel accordingto a second scanning signal of the at least two scanning signals.

The first scanning signal may be supplied to the one pixel after thesecond scanning signal is supplied to the one pixel.

Another aspect is a pixel that is included in an OLED display. The pixelincludes: an OLED; a first transistor that controls a driving currentflowing to the OLED; and a second transistor that transfers a datavoltage to the first transistor according to a scanning signal, whereinthe first transistor includes a first gate electrode that is connectedto the second transistor and a second gate electrode that is connectedto a DC power source.

The first gate electrode and the second gate electrode may beelectrically insulated by a second insulation layer, and the second gateelectrode and a contact electrode that is connected to the first gateelectrode may include a first transistor that is electrically insulatedby a third insulation layer.

The pixel may further include a first transistor in which the DC voltageis higher than the data voltage and is supplied to the second gateelectrode through the DC power source, when the first transistor is ap-MOSFET.

The pixel may further include a first transistor in which the DC voltageis lower than the data voltage and is supplied to the second gateelectrode through the DC power source, when the first transistor is ann-MOSFET.

Another aspect is a transistor that drives an OLED that is included in apixel of an OLED display. The transistor includes: a polysilicon layer;a first insulation layer that is formed in an upper portion of thepolysilicon layer; a first gate electrode that is formed in an upperportion of the first insulation layer; a second insulation layer that isformed in an upper portion of the first gate electrode; and a secondgate electrode that is formed in an upper portion of the secondinsulating layer, wherein the second gate electrode is electricallyinsulated from the first gate electrode and is connected to a DC powersource.

The transistor may further include a third insulation layer that isformed in an upper portion of the second gate electrode.

The second insulation layer may include a first contact hole, the secondgate electrode may include an opening, and the third insulation layermay include a second contact hole, and the transistor may include acontact electrode that is connected to the first gate through the firstcontact hole, the opening, and the second contact hole.

In this way, a gate voltage of a driving transistor can be preciselyadjusted using a second gate electrode that can supply DC power, and anoperation range of an OLED can be easily secured.

Further, by only adding one power line that can precisely adjust a gatevoltage of a driving transistor to an OLED display, an operation rangeof an OLED can be easily secured and thus a drain current can be reducedwithout increasing a channel length of the driving transistor resultingin a narrower pixel area.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an OLED display.

FIG. 2 is a circuit diagram illustrating a circuit of a 6-transistor and1-capacitor pixel.

FIG. 3 is a timing diagram illustrating operation of a 6-transistor and1-capacitor pixel.

FIG. 4 is a top plan view illustrating a 6-transistor and 1-capacitorpixel.

FIG. 5 is a cut-away cross-sectional view illustrating the pixel takenalong line A-B of FIG. 4.

DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS

In the following detailed description, only certain exemplaryembodiments of the described technology have been shown and described,simply by way of illustration. As those skilled in the art wouldrealize, the described embodiments may be modified in various differentways, all without departing from the spirit or scope of the describedtechnology. Accordingly, the drawings and description are to be regardedas illustrative in nature and not restrictive. Like reference numeralsdesignate like elements throughout the specification.

Further, like reference numerals designate like elements in severalexemplary embodiments and are representatively described in the firstexemplary embodiment and elements different from those of the firstexemplary embodiment will be described in other exemplary embodiments.

Further, in the drawings, a size and thickness of each element arerepresented to facilitate understanding and for ease of description, andthe present invention is not limited thereto. In the disclosedembodiments, “connected” covers electrical connection.

In the drawings, the thickness of layers and regions may be exaggeratedfor clarity. In the drawings, in order to facilitate understanding andfor ease of description, thicknesses of some layers and areas areexaggerated. When it is said that any part, such as a layer, film,region, or plate, is positioned on another part, it means the part isdirectly on the other part or above the other part with at least oneintermediate part.

In addition, unless explicitly described to the contrary, the word“comprise” and variations such as “comprises” or “comprising”, will beunderstood to imply the inclusion of stated elements but not theexclusion of any other elements. Further, in the specification, thedescription of an upper part of a target portion indicates an upper partor a lower part of a target portion, and does not mean that the targetportion is always positioned at the upper side based on a direction ofgravity.

FIG. 1 is a diagram illustrating an OLED display.

Referring to FIG. 1, an OLED display includes a display unit 100including a plurality of pixels 10, a scan driver 110, a data driver120, a light emission driver 130, a DC power source unit 140, and acontroller 150.

The pixel 10 that is included in the display unit 100 can receive ascanning signal from the scan driver 110 through two scan lines of aplurality of scan lines.

The pixel 10 can receive a data signal from the data driver 120 throughone data line of a plurality of data lines and can receive a second datasignal from the light emission driver 130 through one light emissioncontrol line of a plurality of light emission control lines.

In an exemplary embodiment of FIG. 1, the pixel 10 is connected to ascan driver through scan lines S[i−1] and S[i], is connected to the datadriver 120 through a data line D[j], and is connected to the lightemission driver 130 through a light emission control line EM[i].

In this case, in an OLED display, the pixel 10 receives a DC voltagethrough the DC power source unit 140. This case will be described indetail with reference to FIGS. 2 to 5.

The controller 150 can receive a video signal, a synchronization signal,and a clock signal from the outside and can transfer a control signal tothe scan driver 110, the data driver 120, the light emission driver 130,and the DC power source unit 140.

FIG. 2 is a circuit diagram illustrating a circuit of a 6-transistor and1-capacitor pixel, and FIG. 3 is a timing diagram illustrating operationof a 6-transistor and 1-capacitor pixel.

Referring to FIG. 2, the pixel 10 includes an OLED that is connectedbetween a first power source ELVDD and a second power source ELVSS. 6thin film transistors (TFT) and one capacitor that are positionedbetween the OLED and the first power source ELVDD to control a drivingcurrent to be supplied to the OLED.

In some embodiments, each pixel 10 that is included in the OLED displayincludes a TFT. Further, a transistor T1 of a TFT that is included inthe pixel 10 further includes a second gate electrode 11 in which DCpower is separately supplied by the DC power source unit 140, unlikeother transistors.

In general, a gate voltage of a driving transistor is determinedaccording to a voltage that is transferred through a switchingtransistor T2 or a voltage that is stored at a storage capacitorC_(strg) and thus it is generally difficult to artificially controloperational characteristics of the driving transistor from the outsideof the pixel.

According to an exemplary embodiment of the described technology, thesecond gate electrode 11 that is further included in the drivingtransistor separately receives a supply of DC power to function as akind of resistor, and thus the second gate electrode 11 easily controlsoperation of the driving transistor from the outside of the pixel. Afunction and operation of the second gate electrode 11 will be describedin detail with reference to FIGS. 4 and 5.

A first scan line and a second scan line are connected to a pixel. InFIG. 2, the scan line S[i] corresponds to the first scan line, and thescan line S[i−1] corresponds to the second scan line.

In some embodiments, the first transistor T1 is a driving transistorthat enables flow of a current to the OLED, and the second transistor T2is a switching transistor that receives a first scanning signal from thefirst scan line to transfer a data voltage to a driving transistor.

In order to compensate a threshold voltage V_(th−) of the drivingtransistor, a third transistor T3 is a transistor that enables adiode-connection of the driving transistor. A fourth transistor T4 is aninitialization transistor that transfers an initialization voltageV_(INT) for an initialization period.

A fifth transistor T5 and a sixth transistor T6 are a light emissioncontrol transistor and receive a light emission control signal from thelight emission driver 130 to control operation of the drivingtransistor. The OLED generates a predetermined luminance of lightcorresponding to a driving current supplied by the driving transistor.

Hereinafter, operation of the pixel circuit of FIG. 2 will be describedin detail with reference to a timing diagram of FIG. 3.

At a time point t₀, when a light emission control signal rises to a highlevel, the fifth transistor T5 and the sixth transistor T6 are turnedoff. Accordingly, a driving current supplied to the OLED is blocked.

Thereafter, a time point t₁, when the second scanning signal S[i−1]falls to a low level, the fourth transistor T4 is turned on, and aninitialization voltage V_(INT) is thus supplied to a gate of the drivingtransistor. That is, a gate voltage V_(G) of the driving transistor isreset to the initialization voltage V_(INT).

Thereafter, at a time point t₂, when the second scanning signal S[i−1]rises to a high level, and at a time point t₃, when the first scanningsignal S[i] falls to a low level, the second transistor T2 and the thirdtransistor T3 are turned on.

In this case, a data voltage V_(DATA) is transferred to a sourceelectrode N2 of the driving transistor through the second transistor T2,and the third transistor T3, which has been turned on, forms diodeconnection in which a gate and a drain of the driving transistor areconnected. Thereafter, a voltage that is subtracted by a thresholdvoltage of the driving transistor from a data voltage that is suppliedto the source of the driving transistor is supplied to one electrode ofthe capacitor C_(strg). Therefore, the capacitor C_(strg) is chargedwith a voltage ELVDD−(V_(DATA)−V_(TH)) corresponding to a differencebetween the first power source ELVDD and a voltage V_(DATA)−V_(TH), thatis a threshold voltage subtracted from the data voltage.

Thereafter, at a time point t₄, when the first scanning signal S[i]rises to a high level, and at a time point t₅, when a light emissioncontrol signal EM[i] falls to a low level, the fifth transistor T5 andthe sixth transistor T6 are turned on. Because the fifth and sixthtransistors T5 and T6, which are a light emission control transistor areturned on, a driving current is transferred to the OLED by a voltagethat is stored at the capacitor C_(strg), and the OLED emits light.

FIG. 4 is a top plan view illustrating a 6-transistor and 1-capacitorpixel, and FIG. 5 is a cut-away cross-sectional view taken along lineA-B illustrating the pixel of FIG. 4.

FIG. 4 is a top plan view of a pixel in which a circuit diagram of FIG.2 may be embodied. Referring to FIG. 4, an initialization voltageV_(INT) is supplied in an x-direction to a pixel, and the first scanningsignal S[i], the second scanning signal S[i−1], and the light emissioncontrol signal EM[i] are transferred to the pixel. Further, the firstpower source ELVDD and the data voltage V_(DATA) are supplied in ay-direction.

Referring to FIGS. 2 and 4, one electrode 402 of the capacitor C_(strg)is connected to the first power source ELVDD in a first conductive areaCOA1, and another electrode 401 is connected to N1 in a secondconductive area COA2. Further, one of a source or a drain of the thirdtransistor T3, a source of the fourth transistor T4, and a gate of thefirst transistor T1 share N1.

Further, a source of the first transistor T1, a drain of the secondtransistor T2, and a drain of the fifth transistor T5 share N2.

Finally, one of a source or a drain of the third transistor T3 differentfrom the one that is connected to the node N1, a drain of the firsttransistor T1, and a source of the sixth transistor T6 share N3.

In some embodiments, as shown in FIG. 4, N1, N2, and N3 are apolysilicon layer 502 (shown in FIG. 5) of the transistor, andimpurities (electrons or holes) are doped to be used as a source or adrain of the transistor. Further, in an upper portion of the polysiliconlayer 502, conductive areas COA1 and COA2 are formed.

In FIG. 4, the OLED and the second power source ELVSS are not shown, butthe sixth transistor T6 is connected to the OLED, and thus until thesixth transistor T6 is turned on, a driving current is supplied from thedriving transistor.

Referring to FIG. 4, a pixel includes a second gate electrode 11 towhich a DC voltage is directly supplied.

Since the second gate electrode 11 covers a first gate electrode 510 ofthe driving transistor and encloses a contact electrode 506, the secondgate electrode 11 is electrically insulated from the contact electrode506. In this case, a portion in which the first gate electrode 510 andthe second gate electrode 11 are not overlapped may be optimizedaccording to a process.

In some embodiments, since a DC voltage is directly supplied to thesecond gate electrode 11 by a DC power source unit, the second gateelectrode 11 functions as a kind of resistor between a gate and a drainof a driving transistor, and a magnitude of a driving current isartificially adjusted from the outside of a pixel.

Hereinafter, operation of a driving transistor including the second gateelectrode 11 will be described in detail with reference to FIG. 5.

FIG. 5 is a cut-away cross-sectional view illustrating the pixel takenalong line A-B of FIG. 4.

Referring to FIG. 5, a driving transistor includes a buffer insulationlayer 501, a polysilicon layer 502 in which electrons or holes aredoped, a first insulation layer 503, a first gate electrode 510, asecond insulation layer 504, a second gate electrode 11, a thirdinsulation layer 505, a contact electrode 506, a first contact hole 507,a second contact hole 508, and an opening 509. In a process ofmanufacturing a driving transistor, for example, a driving transistormay include forming the first gate electrode 510 in an upper portion ofthe first insulation layer 503 and forming the second insulation layer504.

After the second gate electrode 11 is formed in an upper portion of thesecond insulation layer 504, the opening 509 is formed by selectivelyetching an area from the second gate electrode 11, in which the contactelectrode 506 is to be inserted.

Thereafter, in an upper portion of the selectively removed second gateelectrode 11, the third insulation layer 505 is formed. Thereafter, bypartially etching the second insulation layer 504 and the thirdinsulation layer 505, the first contact hole 507 and the second contacthole 508 are formed.

In this case, by inserting the contact electrode 506 into an etchedportion, a voltage is supplied to the first gate electrode 510 throughthe contact electrode 506. That is, the contact electrode 506 isconnected to the first gate electrode 510 through the first contact hole507, the opening 509, and the second contact hole 508.

The second gate electrode 11 is electrically insulated from the firstgate electrode 510 by the second insulation layer 504 and iselectrically insulated from the contact electrode 506 by the thirdinsulation layer 505.

In the foregoing description, a process of manufacturing a drivingtransistor has been described, however, a method of manufacturing adriving transistor of the described technology is not limited to theabove-described method.

According to an exemplary embodiment of the described technology, thefirst gate electrode 510 receives a supply of a voltage from the contactelectrode 506, and the second gate electrode 11 independently receives asupply of a voltage from the DC power source unit 140, and thus whenusing the second gate electrode 11, a gate voltage of a drivingtransistor may be formed regardless of a voltage of the first gateelectrode 510.

That is, by supplying a DC voltage to the second gate electrode 11, aresistive effect can be obtained between a gate and a drain of thedriving transistor and thus a drain current I_(−d) may be reducedlargely securing an operation range of an OLED without increasing achannel length of the driving transistor.

In one embodiment, when the driving transistor is a p-MOSFET, if thedriving transistor is operated, a DC voltage higher than a voltage thatis supplied to the first gate electrode 510 is supplied to the secondgate electrode 11 and thus the drain current I_(−d) of the drivingtransistor may be reduced.

In another embodiment, when the driving transistor is an n-MOSFET, ifthe driving transistor is operated, a DC voltage lower than a voltagethat is supplied to the first gate electrode 510 is supplied to thesecond gate electrode 11 and thus the drain current L_(d) of the drivingtransistor may be reduced.

In these embodiments, a voltage that is supplied to the second gateelectrode 11 may be supplied according to whether the driving transistoris a p-MOSFET or an n-MOSFET and a magnitude of a supply voltage ischanged according to a design condition.

According to at least one of the disclosed embodiments, by including thesecond gate electrode 11 that may independently supply a DC voltage in adriving transistor, a gate voltage of the driving transistor may beprecisely adjusted securing an operation range of the OLED.

Further, by adding one power line that can precisely adjust a gatevoltage of the driving transistor to a pixel circuit, an operation rangeof the OLED can be easily secured and thus the drain current L_(d) maybe reduced without increasing a channel length of the driving transistorresulting in a narrower pixel area. Therefore, a pixel circuit canadvantageously embody a high resolution OLED display.

The above description is for illustrative purposes only and is notintended to be limited to the disclosed embodiments, but, on thecontrary, is intended to cover various modifications and equivalentarrangements included within the spirit and scope of the accompanyingclaims.

What is claimed is:
 1. An organic light-emitting diode (OLED) display,comprising: a display unit comprising a plurality of pixels, whereineach of the pixels comprises at least one thin film transistor (TFT), atleast one capacitor, and at least one OLED; a scan driver configured totransfer a plurality of scanning signals to the display unit; a lightemission driver configured to transfer a plurality of light-emittingsignals to the display unit; a data driver configured to transfer aplurality of data signals to the display unit; a DC power source unitconfigured to transfer direct current (DC) power to the display unit;and a controller configured to control the scan driver, the lightemission driver, the data driver, and the DC power source unit, whereinthe at least one TFT comprises a driving transistor configured tocontrol a driving current flowing to the OLED, wherein the drivingtransistor comprises a first gate electrode formed over a firstinsulation layer and configured to receive a data voltage from the datadriver and a second gate electrode configured to receive a DC voltagefrom the DC power source unit, wherein the OLED display furthercomprises a second insulation layer that is interposed between and indirect physical contact with the first and second gate electrodes,wherein a contact electrode is electrically connected to the first gateelectrode, wherein the contact electrode is electrically insulated tothe second gate electrode by a third insulation layer, wherein a firstcontact hole is formed in the second insulation layer, wherein a secondcontact hole is formed in the third insulation layer, and wherein aportion of the contact electrode is inserted into the first and secondcontact holes and is in direct physical contact with the first gateelectrode.
 2. The OLED display of claim 1, wherein the drivingtransistor is a p-MOSFET, and wherein the DC voltage supplied to thesecond gate electrode of the driving transistor is higher than the datavoltage.
 3. The OLED display of claim 1, where the driving transistor isan n-MOSFET and wherein the DC voltage supplied to the gate electrode ofthe driving transistor is lower than the data voltage.
 4. The OLEDdisplay of claim 1, wherein the scan driver is configured to i) supplyat least two of the scanning signals to one of the pixels, wherein theat least two scanning signals comprise first and second scanningsignals, ii) supply one of the data signals to the one pixel accordingto the first scanning signal, and iii) supply an initialization voltageto the one pixel according to the second scanning signal.
 5. The OLEDdisplay of claim 4, wherein the scan driver is further configured tosupply the first scanning signal to the one pixel after the secondscanning signal is supplied to the one pixel.
 6. The OLED display ofclaim 1, wherein a first contact hole is formed in the second insulationlayer, wherein the driving transistor further comprises a contactelectrode that is inserted into the first contact hole and is in directphysical contact with the first gate electrode, and wherein the contactelectrode does not directly contact the second gate electrode.
 7. Apixel for organic light-emitting diode (OLED) display, the pixelcomprising: an OLED; a first transistor configured to control a drivingcurrent flowing to the OLED; and a second transistor configured totransfer a data voltage to the first transistor according to a scanningsignal, wherein the first transistor comprises a first gate electrodeformed over a first insulation layer and electrically connected to thesecond transistor and a second gate electrode electrically connected toa DC power source, wherein the OLED display further comprises a secondinsulation layer that is interposed between and in direct physicalcontact with the first and second gate electrodes, wherein the firsttransistor further comprises a contact electrode that is electricallyconnected to the first gate electrode wherein the first transistor iselectrically insulated b a third insulation layer, wherein a firstcontact hole is formed in the second insulation layer, wherein a secondcontact hole is formed in the third insulation layer, and wherein aportion of the contact electrode is inserted into the first and secondcontact holes and is in direct physical contact with the first gateelectrode.
 8. The pixel of claim 7, wherein the first transistor is ap-MOSFET, and wherein the DC voltage supplied to the second gate of thefirst transistor is higher than the data voltage.
 9. The pixel of claim7, wherein the first transistor is an n-MOSFET, and wherein the DCvoltage supplied to the second gate of the first transistor is lowerthan the data voltage.
 10. A transistor for driving a pixel of anorganic light-emitting diode (OLED) display, the transistor comprising:a polysilicon layer; a first insulation layer formed over thepolysilicon layer; a first gate electrode formed over the firstinsulation layer; a second insulation layer formed over the first gateelectrode; and a second gate electrode formed over the second insulatinglayer, wherein the second insulation layer is interposed between and indirect physical contact with the first and second gate electrodes, andwherein the second gate electrode is electrically insulated from thefirst gate electrode and is electrically connected to a DC power source,wherein the second insulation layer comprises a first contact hole,wherein the second gate electrode comprises an opening, wherein thethird insulation layer comprises a second contact hole, and wherein thetransistor comprises a contact electrode electrically connected to thefirst gate through the first contact hole, the opening, and the secondcontact hole.
 11. The transistor of claim 10, further comprising a thirdinsulation layer formed over the second gate electrode.
 12. A displaydevice, comprising: a plurality of pixels; a plurality of data linescomprising first and second data lines; and a direct current (DC) powersource, wherein each of the pixels comprises at least one thin filmtransistor (TFT) and at least one organic light-emitting diode (OLED),wherein the at least one TFT comprises a driving transistor configuredto control a driving current flowing to the OLED, wherein the drivingtransistor comprises a first gate electrode formed over a firstinsulation layer and configured to receive a first data signal from thefirst data line and a second gate electrode configured to receive a DCvoltage from the DC power source, wherein the display device furthercomprises a second insulation layer that is interposed between and indirect physical contact with the first and second gate electrodes,wherein the driving transistor further comprises a contact electrodethat is electrically connected to the first gate electrode, wherein thedriving transistor is electrically insulated by a third insulationlayer, wherein a first contact hole is formed in the second insulationlayer, wherein a second contact hole is formed in the third insulationlayer, and wherein a portion of the contact electrode is inserted intothe first and second contact holes and is in direct physical contactwith the first gate electrode.
 13. The display device of claim 12,further comprising at least two scan lines that include first and secondscan lines, wherein a source electrode of the driving transistor isconfigured to receive a second data signal from the second data lineaccording to a first scanning signal received from the first scan line,and wherein the first gate electrode is configured to receive the firstdata signal according to a second scanning signal received from thesecond scan line.
 14. The display device of claim 13, furthercomprising: a data driver configured to apply a plurality of datasignals to the data lines; a scan driver configured to apply a pluralityof scan signals to the at least two scan lines, and a controllerconfigured to control the data driver and the scan driver.
 15. Thedisplay devices of claim 12, wherein the at least one TFT furthercomprises a diode connection transistor electrically connected to form adiode connection between a drain electrode of the driving transistor andat least one of the first and second gate electrodes of the drivingtransistor.
 16. The display device of claim 15, wherein the diodeconnection further comprises: an electrical connection between a sourceelectrode of the diode connection transistor and the drain electrode ofthe driving transistor, and a second electrical connection between adrain electrode of the diode connection transistor and the at least onegate electrode of the driving transistor.
 17. The display device ofclaim 15, wherein the diode connection further comprises: an electricalconnection between a drain electrode of the diode connection transistorand the drain electrode of the driving transistor, and a secondelectrical connection between a source electrode of the diode connectiontransistor and the at least one gate electrode of the drivingtransistor.